Case In point I own 10 acres of land in the USA and find the world biggest oil reserve or the world biggest Uranium reserve and I really really hate Britain for whatever reason you can pick. Does that give the UK to enter and bomb my 10 acres back to the stone age?
Aside from those in the nuclear club not wanting to share power, nuclear proliferation causes a lot of problems. It creates more flashpoints for escalating into a world-ending nuclear exchange or for non-state actors to obtain devices for terrorism, etc. The world stood at the brink for a while and decided decades ago the ultimate goal is to reduce the number of nuclear arms.
On the flip side, this is why the recent-ish invasions of Afghanistan, Iraq, and Venezuela, the intervention in Libya, and China’s saber-rattling against Taiwan–among others–are serious matters. It teaches regimes around the world you will only be safe if you have a nuclear deterrent. North Korea applied that lesson, and they won’t be the last at this rate.
And that’s why we know Iran has no nukes